1. Field of the Invention
The present invention relates to a support structure for a planet gear in a simple planetary gear mechanism.
2. Description of the Related Art
The simple planetary gear mechanism comprises a sun gear; a planet gear which is supported by a carrier pin that is in turn supported by a carrier and which circumscribes the sun gear; and an internal gear in which the planet gear is inscribed. The simple planetary gear mechanism is compact, capable of providing a reduction ratio, and is adapted to easily change the reduction ratio (or the speed increasing ratio) by changing the arrangement and the input and output of each component. Thus, the simple planetary gear mechanism is widely used in various applications.
In recent years, on the other hand, production lines in factories have increased in complexity to manufacture many items in the same line. Accordingly, the entire line is not powered with a single power source as in the past but a special motor with a speed reducer is placed at individual portions of the line to cope with product diversification as well as to increase the efficiency of production. In this context, attention has been given to the simple planetary gear mechanism, as a type of a speed reducer coupled to the motor, which can be made compact.
However, the presence of backlash in each gear has turned into problems in such applications as the robot arm that requires forward and reverse rotations repeatedly and precise positioning of the arm at the predetermined absolute positions. That is, there exists backlash in the gear engagement in an unavoidable manner. With an excessive backlash, such a problem may arise that a command for reverse rotation cannot be immediately reflected on the reverse rotation of an input member (e.g., a carrier) even when the sun gear has already started its reverse rotation. This may also cause the arm to be out of alignment with its stopping positions. Accordingly, for a speed reducer of this type, the reduction of backlash has been a big problem to be tackled. The most basic method for reducing backlash has been actually practiced in many fields. According to the method, a plurality of gears are prepared first which have dimensions slightly different from each other, and then a gear is selected which engages another with the least gap upon assembly. In general, since many planet gears are employed in a speed reducer and therefore not selected preferably by try and error, internal and sun gears are to be selected.
However, there is a problem in this method that requires a large quantity of stocks and significantly reduces the efficiency of assembly, thereby easily resulting in an increase in cost. That is, for the simple planetary gear mechanism, there is another problem of variations in support position of the carrier pin relative to the carrier in addition to variations in dimension of each gear (i.e., the sun gear, the planet gear, and the internal gear). Accordingly, for example, suppose that the planet gear is given a dimension smaller than its standard dimension. In this case, to obtain proper backlash by the aforementioned method, it is necessary to incorporate a sun gear having a dimension greater than its standard dimension and an internal gear having a dimension less than its standard dimension.
Furthermore, suppose that each gear has successfully engaged another with no backlash at a certain position. Even in this case, when an attempt is made to actually rotate the gears, the gears may interfere with another at other position (due to the reduced backlash) to significantly increase the resistance to rotation. This has required a try and error attempt involving tremendous time and costs in order to provide the gears with the minimized backlash for smooth rotation at any time and positions.
In this context, there have been suggested several techniques for reducing the backlash not by try and error but by a simpler method in which some thought is put into the structure of the mechanism.
Japanese Patent Laid-Open Publication No. Sho 63-6248 suggested the following method for realizing reduced backlash. According to this method, in a simple planetary gear mechanism comprising four planet gears, those facing to each other are paired and a carrier pin for one pair of planet gears is offset in the circumferential direction in a so-called scissors configuration.
Japanese Patent Laid-Open Publication No. Hei 052-40315 also suggested the following method for realizing reduced backlash. According to this method, in a simple planetary gear mechanism comprising four planet gears, those facing to each other are paired and a carrier pin for one pair of planet gears is offset in the axial direction. In addition to this, the internal gear is divided into two in the axial direction to the offset planet gears and then one side of the internal gear is twisted in the circumferential direction to form a scissors configuration. One of the carriers disposed on both sides provides an output of forward rotation and the other provides an output of reverse rotation.
Japanese Patent Laid-Open Publication No. Hei 08-61438 suggested a structure for providing reduced backlash, in which the internal gear of the simple planetary gear mechanism is formed of elastic material to provide a negative backlash caused by the engagement of the internal gear with the planet gear.
However, consider a method for reducing backlash such as by the structure disclosed in Japanese Patent Laid-Open Publication No. Sho 63-6248. This method makes it possible to positively reduce the backlash in both the forward and reverse rotations. However, the method presents a problem that the amount of transmitted torque is reduced by one-half because only two planet gears contribute to the transmission of drive at the time of forward and reverse rotations, respectively, even though the mechanism is provided with four planet gears. There is also another problem that this method has a low degree of flexibility in designing the mechanism. That is, to maintain the torque capacities of forward and reverse rotations at the same level, it is necessary to assign the same number of planet gears to each rotation. This restricts the total number of planet gears to four in practice.
In this connection, it is well known that the so-called three-point support is better than the four-point support from the viewpoint of absorbing (or permitting) the dimensional or assembling error of each gear. However, this technique employs the arrangement in which the planet gears facing to each other are paired to adjust the backlash. This arrangement also raises a problem that it is not possible to employ a structure comprising only three planet gears, the structure corresponding to the three-point support.
Similarly, consider a method for reducing backlash such as by the structure disclosed in Japanese Patent Laid-Open Publication No. Hei 05-240315. This method also presents a problem that drive can be transmitted only by two of the four planet gears and therefore the transmission torque capacity cannot be maintained at high levels. The method raises another problem that it is not possible to employ a structure with the three-point support.
In addition, consider a method for reducing backlash such as by the structure disclosed in Japanese Patent Laid-Open Publication No. Hei 08-61438. This method provides an advantage of allowing all the planet gears to contribute to both the forward and reverse rotations. Nevertheless, the method requires the formation of the internal gear of an elastic material, thus providing bad machinability and making it difficult to maintain the strength at a sufficient level.
This method presents another problem of providing disadvantageous durability caused by the internal gear deformed each time the engagement position between the internal gear and each planet gear is displaced.
The present invention was developed in view of these conventional problems. It is therefore the main object of the present invention to allow all planet gears to contribute to both forward and reverse rotations and implement assembly in the minimized backlash condition at any position of engagement through a simple procedure. It is also another object of the present invention to provide a related method for manufacturing the mechanism.
The present invention solves the aforementioned problems by employing the following support structure for a planet gear in a simple planetary gear mechanism. The mechanism includes a sun gear, a planet gear, supported by a carrier pin supported by a carrier, circumscribing the sun gear, and an internal gear inscribed by the planet gear. The mechanism is adapted such that the carrier pin is provided with a crank portion having an eccentric direction, eccentric with respect to an axis center of the carrier pin, the eccentric direction being radially variable. The mechanism is also adapted such that the planet gear is supported rotatably about an axis center of the crank portion, the axis center being radially variable.
According to the present invention, the carrier pin for supporting the planet gear is not of a simple cylindrical shape as in the prior art but provided with the crank portion having an eccentric direction, eccentric with respect to the axis center of the carrier pin, the eccentric direction (an eccentric angle xcex1) being radially variable. More precisely, the eccentric angle xcex1 is xe2x80x9can angle formed between the tangential direction (aligned with an eccentric reference direction) of the axis center of carrier pin in the simple planetary gear mechanism and a straight line connecting between the axis center of the carrier pin and the axis center of the crank portion.xe2x80x9d A variation in the eccentric angle xcex1 causes the axis center of the crank portion to vary in the radial direction. The planet gear is supported rotatably about the axis center of the crank portion that is made variable in the radial direction.
Basically, backlash is xe2x80x9cplayxe2x80x9d that is present between teeth of gears in the circumferential direction. This concept has lead to a technique for reducing the backlash by shifting the engaging teeth xe2x80x9cin the circumferential directionxe2x80x9d relative to each other, and this technique has been dominant for a long time. In contrast to this technique, the present invention is particularly characterized in that the axis center is made variable xe2x80x9cin the radial direction.xe2x80x9d
This arrangement allows the axis center of the crank portion for actually rotatably supporting the planet gear (or the substantial center of rotation of the planet gear) to be set at the best balanced position in relation to the internal gear or the sun gear. This has made it possible to cope with a variety of manufacturing variations in an extremely flexible manner. With this arrangement, for example, it is possible to absorb the manufacturing error of each gear at a time including the displacement of the carrier pin of a planet gear by preparing only a plurality of sun gears having dimensions slightly different from each other and then simply selecting a sun gear having a proper dimension upon its assembly.
Among other things, it is made possible to allow all the planet gears to contribute to the transmission of drive both in the forward and reverse rotations. For example, since either an odd or even number of planet gears can be employed, it is also possible to provide an advantageous xe2x80x9cthree-point support structurexe2x80x9d using three planet gears, as described later.
Incidentally, the present invention is adapted to adjust the support position of a planet gear in the radial direction. For example, when the planet gear is brought towards the internal gear in this arrangement, the sun gear will be provided with increased backlash in principle. For this reason, it is preferable to adjust the backlash on the side of the sun gear. However, the present invention does not always make it inevitable to xe2x80x9cselect and adjustxe2x80x9d the gear having an expanded backlash for the purpose of assembly. On the contrary, the present invention can still provide a sufficient advantage even in a case where all gears are manufactured at their predetermined design dimensions and incorporated as they are.
That is, the axis center of a planet gear was usually fixed and the effect of an error in the position of the axis center had to be taken into account. Thus, for example, it could not be avoided to make the design dimension of the sun gear smaller and the design dimension of the internal gear larger. However, the present invention provides such a planet gear as has an automatic alignment function, thereby making an allowance for machining tolerances. This provides an advantage of reducing the tendency of the machining tolerance to have an adverse effect even when the design backlash is set at a smaller value.
Furthermore, the planet gear is positioned where the active and reactive forces against both the internal gear and the sun gear are kept in balance. This makes it possible to provide an effect of allowing for very smooth rotations even with the backlash minimized.
Incidentally, according to the present invention, no particular restrictions are imposed on how to specifically form the crank portion, but various arrangements can be employed.
For example, it is possible to form the crank portion of the carrier pin by providing an eccentric cam, capable of rotatably supporting the planet gear, integrated with the carrier pin in the direction of rotation, and by rotatably supporting the carrier pin by means of the carrier. For the xe2x80x9cintegration in the rotational directionxe2x80x9d, the carrier pin and the eccentric cam may be formed completely in one piece or a plurality of members may be combined together to be consequently integrated in the rotational direction. For the xe2x80x9crotatable supportxe2x80x9d, a bearing or the like may be interposed between the planet gear and the carrier pin.
Furthermore, the eccentric cam capable of rotatably supporting the planet gear can be rotatably incorporated onto the outer circumference of the carrier pin to thereby form the crank portion of the carrier pin.
Incidentally, for example, it is preferable to employ three planet gears for a more rational application of the present invention.
This is because the planet gear needs to rotate, while rotating about its own axis, on an orbit that is limited to the inside of the internal gear and the outside of the sun gear. Thus, from the viewpoint of absorbing the dimensional and assembling errors of each gear, the arrangement comprising three planet gears, which corresponds to the so-called three-point support, is the most rational one.
According to the present invention, all the planet gears can contribute to the transmission of torque and it is not necessary to provide an even number of planet gears. Thus, from the viewpoints of the torque transmission capacity and the number of planet gears required to reduce backlash, it is possible to set the number of planet gears at xe2x80x9c3xe2x80x9d without any problem.
Incidentally, the eccentric reference direction of the crank portion of each of the carrier pins may be preferably directed towards the same side relative to a straight line connecting between the axis center of the simple planetary gear mechanism and the axis center of the carrier pin.
The present invention provides a planet gear configured such that the axis center of the carrier pin is eccentric with respect to the center of rotation of the planet gear, which may cause a moment (a radial force) to be produced for pushing or pulling the planet gear outwardly or inwardly. However, it has been found that an adverse effect exerted by this moment can be reduced in many cases by maintaining the eccentric reference direction of the carrier pin of all the planet gears to the same side upon assembly.
The xe2x80x9ceccentric reference directionxe2x80x9d means the direction in which the axis center of the crank portion is eccentric with respect to the axis center of the carrier pin, or a reference direction of eccentricity. More specifically, this direction is aligned (but not necessarily coincident perfectly) with the tangent of the carrier pin. Then, with respect to this eccentric reference direction, the crank portion is rocked about the axis center of the carrier pin, thereby making the axis center of the crank portion variable in the radial direction.
Now, consider the case where xe2x80x9cthe eccentric reference direction is directed to the same side with respect to the straight line connecting between the axis center of the simple planetary gear mechanism and the axis center of the carrier pin.xe2x80x9d As described above, this means that it takes place in an unified manner whether the axis center of the crank portion is eccentric towards the forward or reverse direction of rotation with respect to the axis center of the carrier pin.
Furthermore, concerning the occurrence of the moment, the difference in the moment between the forward and reverse directions of rotation should be reduced to an unnoticeable level. For this purpose, an even number of the planet gears may be provided intentionally. Then, one eccentric reference direction of the crank portion of adjacent carrier pins is alternately directed opposite to the other relative to the straight line connecting between the axis center of the simple planetary gear mechanism and the axis center of the carrier pin.
That is, the force that pushes or pulls the planet gear outwardly or inwardly is reversed at the time of forward and reverse rotations.
Therefore, to reduce the difference between the effects of the moment produced at the time of forward and reverse rotations, it is the most rational to assemble the planet gear such that the behavior at the time of the forward rotation is likely equal to that at the time of the reverse rotation.
Incidentally, it has been confirmed that these moments can be reduced to a negligible level in practice by putting some thought into design because the manufacturing errors of the gears and the support hole for the carrier pin are subtle and the range of the errors can be specified.
Incidentally, the most significant feature of the present invention is the automatic alignment function provided for the crank portion of the carrier pin. For example, this function provides an advantage that even during running, the relationship of torque (or radial torque) among the internal gear, the planet gear, and the sun gear can be kept in balance in real time. Suppose that the sun gear is incorporated in a floated condition (in which its axis center can vary in a subtle manner relative to the axis center of the speed reducer, e.g., in a coupled condition by means of splines or means described later). In this case, the radial torques of all the planet gears are kept in balance in real time. In particular, with three planet gears, the speed reducer can be operated theoretically with the radial torques of all the planet gears kept in balance at all times, which could be a great merit depending on its application.
However, on the other hand, this automatic alignment function involves radial moments produced during running as described above. For example, when used for applications that require rotations at high speeds, the speed reducer would be affected by subtle manufacturing variations and thus its smooth rotation and support may be obstructed.
In this context, it should be a good idea to configure the speed reducer such that the eccentric direction of the crank portion of the carrier pin can be fixed in order to intentionally disable this automatic alignment function, once the speed reducer has been completely assembled under optimum conditions.
According to the present invention, when the eccentric direction of the crank portion of the carrier pin has been fixed, the axis center of the crank portion under the fixed condition has the same configuration as that of a xe2x80x9ctypicalxe2x80x9d simple planetary gear mechanism corresponding to the conventional axis center of a carrier pin. In other words, this means that the adjustment of only one sun gear can easily realize the same configuration as that of a simple planetary gear mechanism which is optimized conventionally by selecting components having an optimum dimension by try and error.
As described above, since the planet gear can be provided with the automatic alignment function, the present invention can readily manufacture a simple planetary gear mechanism having a minimized backlash by the following method. That is, the method includes the steps of first incorporating the internal gear and the planet gear, selectively extracting a sun gear having a proper dimension from a plurality of sun gears having different dimensions, and then incorporating the extracted sun gear into the central portion of the already incorporated planet gears while the eccentric direction of the crank portion of the carrier pin is being adjusted.
Incidentally, in principle, the initial object of the present invention can be achieved superficially if the crank portion is adapted to be rockable with respect to the axis center of the carrier pin and thereby the axis center of the planet gear is variable in the radial direction.
However, it has been confirmed that various drawbacks could occur when the position of the axis center of the carrier pin is not properly set upon designing and manufacturing a planet gear speed reducer to which the present invention is actually applied.
For example, as described above, the present invention provides each planet gear configured such that the axis center of the carrier pin is eccentric with respect to the center of rotation of the planet gear, which may cause a moment (a radial force) to be produced for pushing or pulling the planet gear outwardly or inwardly. This moment obstructs the smooth rotation of the planet gear and causes loss of energy, therefore reducing efficiency and thereby causing unnecessary heat to be generated. It is thus desirable to reduce this moment to a minimum, however, it has been found that the magnitude of the moment depends largely on the position to which the axis center of the carrier pin is set.
This finding has made it possible to set the axis center of the carrier pin at the optimum position, thereby making it possible to minimize the surfacing of the potential drawbacks of the present invention (described later).
In addition, the concept of the present invention is almost applicable not only to the simple planetary gear mechanism but also to a simple planetary roller mechanism including a sun roller, a planet roller, a ring roller (corresponding the internal gear) and a carrier supporting the planet roller. Therefore, the present invention includes the concept of the simple planetary roller mechanism despite the wording xe2x80x9cgearxe2x80x9d in the accompanied claims without departing from the basic sprit of the claimed invention.
As described above, the present invention can provide an automatic alignment function for the planet gear (a planet roller), and this automatic alignment function serves to improve the smoothness during running irrespective of various dimensional or assembly errors. Furthermore, it is also possible to employ a simple method for selectively incorporating a sun gear (or a sun roller) having a proper dimension by making use of this automatic alignment function. This method makes it possible to provide a speed reducer which has a reduced backlash or which is pre-pressurized appropriately.